The invention relates to a cleaning solution, and particularly to a cleaning solution for removing metallic contaminants of substrates to which a metallic wiring has been applied.
More particularly, the invention relates to a cleaning solution for substrates having metallic wiring used after a chemical mechanical polishing (CMP) process in semiconductor manufacturing.
A trend for higher integration of ICs has brought about the need for strict contamination control, because trace impurities have a considerable effect on the properties and yield of devices. Which means that the concentration of metallic impurities at the substrate surface needs to be limited to or below 10.sup.10 atoms/cm.sup.2 ; for this purpose different types of cleaning solutions are used in the different processes of semiconductor manufacturing.
In general, mixed solutions of sulfuric acid-aqueous hydrogen peroxide, ammonium water-aqueous hydrogen peroxide-water (SC-1), hydrochloric acid-aqueous hydrogen peroxide-water (SC-2), diluted hydrofluoric acid, etc., are used as cleaning solutions for semiconductor substrates, and depending on the purpose, each cleaning solution can be used on its own or in combination with other cleaning solutions. On the other hand, since in recent years CMP technology has been introduced into semiconductor manufacturing processes for the planarization of the insulation film, the planarization of the contact holes, for damascene wiring, etc.; e.g., the monthly publication Semiconductor World, p. 92, 3, 1997, mentions the use of an aqueous solution of citric acid, and WO 96/26538 mentions the use of an aqueous solution of citric acid or of ethylenediamine tetraacetic acid (EDTA), etc., together with hydrogen fluoride, for the purpose of cleaning the metallic impurities adsorbed by the substrate surface after a CMP process. Further, in JP, A, H10-72594 (EP, A, 812011), filed before and laid-open after the priority date of the present application, a cleaning solution containing an organic acid, such as citric acid, etc., and a complexing solution are mentioned.
Since the aqueous solutions, as mentioned above, used in general as a cleaning solution for cleaning a substrate surface, to which a metallic wiring has been applied, are oxidizable and at the same time strongly acidic or strongly alkaline liquids, the metal exposed on said substrate surface is of course corroded (by etching), and even if the metal is covered by an insulating layer, etc., the metal will be corroded when the cleaning solution penetrates the layer; consequently, there are cases when these cleaning solutions cannot be used in semiconductor manufacturing processes. It is, e.g., impossible to utilize such cleaning solutions with applied CMP process such as the layer interconnection technology known as W (tungsten) plug process, which has received much attention recently, etc.
In general, metal CMP is a technology for planarizing a film by polishing the interlayer insulation film and the metallic material by simultaneously using the chemical and physical effects, when a wafer is pressed against a cloth called buff and is rotated while feeding a slurry, a mixture of abrasive particles and chemicals; however, there is a problem that, as a result of the polishing process, a great number of metallic impurities are adsorbed by the surface of the silicon oxide film, etc. Consequently, a cleaning solution for effectively removing such metallic impurities is required.
Especially with W-CMP, a slurry of alumina-iron(III) nitrate is generally used, which has excellent characteristics with regard to polishing speed and processing accuracy; however, the problem arises that a great amount of Fe is adsorbed by the exposed surface of the silicon oxide film after the blanket tungsten film is removed, because iron(III) nitrate is used as oxidizer. And since a high concentration of iron(III) nitrate is used, the Fe surface concentration adsorbed by the surface of the silicon oxide film is, with 10.sup.14 atoms/cm.sup.2 or more, extremely high; besides, secondary contamination to the wafer production line also presents a problem. Consequently, it is preferred that the Fe adsorbed by the substrate during the CMP process be removed to a concentration of 10.sup.10 atoms/cm.sup.2 before passing to the next process. Even though one may hope to remove the metallic impurities with cleaning solutions containing the hydrogen fluoride according to the before-mentioned prior art, it is impossible to solve the problem of metal corrosion and of the etching of the interlayer insulation film.
On the other hand, when applying a method using an aqueous solution of citric acid according to the before-mentioned prior art as cleaning solution after the W plug CMP process, etc., to solve the problem of metal corrosion, even though the corrosion of the metal can be avoided after the cleaning process with an aqueous solution of citric acid, the Fe concentration of the substrate surface is about 10.sup.13 atoms/cm.sup.2, which is not at all sufficient.
Moreover, since a high concentration of 20 to 30% is necessary to obtain an adequate cleaning effect with a cleaning solution using citric acid, the strains on the environment, due to the liquid waste disposal, etc., increase, and shelf life also becomes a problem since the tendency to mold increases.
Thus, a means for solving the above-mentioned problems in a comprehensive manner is not known at present.
Consequently, the object of the present invention is to provide a cleaning solution for cleaning a substrate after metallic wiring has been implemented, capable of removing metallic impurities of a substrate surface with ease and efficiency, without causing metal to corrode, and without creating a problem for the environment or the shelf life.